The distillation column is a unit device widely used in the petroleum refining industry. For some fractionations of heavy oils, for example, fractionating light fraction oil from crude oil, wax oil and the like, the distillation column generally has a relative high bottom temperature, and the heat source for the reboiler has a high heat grade and is not easy to obtain. Since heavy oils tend to thermally crack at high temperatures, therefore, the distillation columns for crude oil or heavy oil generally don't provide with reboiler, and the heat required by distillation is almost provided by the feedstocks. After preheating feedstocks, the fraction oils are vaporized, the vaporized fraction oils are distilled off from the top and/or the sideline of the column, and non-vaporized streams are distilled off from the bottom of the column. The typical fractionation process for example comprises the atmospheric distillation and the vacuum distillation of crude oil.
The crude and vacuum distillation is the first step of the petroleum refining process, and provide with the starting materials for the subsequent processing units in the refinery, and directly with some final products. The fundamental procedure for the crude oil distillation (by example of fuel oil type) comprises heating the crude oil to about 220-260° C. and then sending it to a primary distillation column. Usually, only one top product, i.e. a reforming feedstock or a light gasoline fraction, is cut from the primary distillation column. In some primary distillation columns, besides the top product, a sideline product is cut off. The primary distillation column bottom oil is sent to the atmospheric column.
The conventional procedure for the atmospheric column is shown in FIG. 1. A primary distillation column bottom oil is heat-exchanged or heated by an atmospheric heating furnace 2 to be partially vaporized, and sent to an atmospheric distillation column 8 through an oil transfer line 7. Lighter components are vaporized in the vaporization section of the distillation column, rise up into the fractionation stage, and are condensed by reflux liquid and removed as top or sideline product to produce fraction oils. Non-vaporized streams flow downwards into the stripping stage, and contacted with steam coming from the column bottom on the column plates of the stripping stage. Non-vaporized lighter fractions are stripped out and rise up along steam into the fractionation stage. Lighter components such as gasoline, kerosene, diesel, heavy diesel are obtained. Non-vaporized streams fall into the column bottom and are removed as atmospheric residue.
The conventional procedure for the vacuum distillation is shown in FIG. 3. Atmospheric residue is heated by a vacuum heating furnace 2 to be partially vaporized, and sent to a vacuum distillation column 6 through an oil transfer line 7. Lighter components are vaporized in the vaporization section of the vacuum distillation column, rise up into the fractionation stage, and are condensed by reflux liquid and removed as top or sideline product to produce fraction oils. Non-vaporized streams are removed from the column bottom as vacuum residue.
The design and operation of the crude oil distillation unit will have a great effect on the product quality, the product yield and the economical benefit of the refinery. In the provision of the qualified product, increasing the distillation yield of the atmospheric distillation plant so that lighter components are distilled off in the atmospheric distillation column as much as possible and are not sent to the vacuum distillation any more, in one hand, can produce more lighter fractions, and in the other hand, can reduce the loads on the vacuum heating furnace and the vacuum distillation column; increasing the distillation yield of the vacuum distillation plant can increase the yield of fraction oils, and provide more feedstock for catalytic cracking and hydro-cracking so as to improve the economical benefit of the refinery.
The important factors which influence the yield of fraction oils of the atmospheric and vacuum distillation unit are the temperature and oil-vapor partial pressure of the vaporization section in the distillation column. The higher the temperature of the vaporization section is and the lower the oil vapor partial pressure is, the higher the vaporization ratio is, and therefore the higher the distillation yield of fraction oils is.
Currently, there are two methods for reducing the pressure of the vaporization section in the industry: one is to reduce the column top pressure of the distillation column. For the atmospheric distillation column, it is generally to reduce the pressure drops in the column top oil vapor pipeline and the condensing cooler. For the vacuum distillation column, a vacuum pumping equipment of high performance can effectively reduce the column top pressure. The other is to use packings, tower trays and column internals of high performances to effectively reduce the in-column resistance and so as to remarkably reduce the pressure of the vaporization section.
Another approach of increasing the fraction oil yield of the distillation column is to increase the temperature of the vaporization section. The temperature of the vaporization section is influenced by the outlet temperature of the heating furnace. The higher the outlet temperature of the heating furnace is, the higher the temperature of the vaporization section is. However, the temperature of the heating furnace cannot be too high, this is because it is possible for heavy oils to crack at a temperature higher than 360° C., and the coke produced by cracking the oils will badly influence the long and stable run of the plant. Therefore, a heating furnace with a furnace tube having a stepwise increased diameter and an oil transfer line with a large diameter are generally used industrially so as to reduce the outlet pressure of the heating furnace as much as possible, and therefore reduce the temperature of feedstock in the heating furnace with the proviso of ensuring the vaporization ratio of the feedstock.
Currently, the vacuum column top pressure of the industrial plant has reached as low as 1 kPa (abs.), the pressure of the feeding section has reached as low as 3 kPa (abs.), and therefore it is hardly to further reduce the pressure. It is also more and more difficult to improve the performances of the packings and internals, and the cost is sharply increased. There are also some limitations for using the heating furnace with a furnace tube having a stepwise increased diameter and the oil transfer line with a large diameter. One limitation is that increasing the diameter of the furnace tube should be designed rationally according to the properties of the feedstock oil and the characteristics of the heating furnace, while it is very hard for a delicate furnace tube design due to a wide variety of feedstocks. Another limitation is that since a large quantity of feedstock is vaporized in the furnace tube, the density of the feedstock in the tube continuously decreases, especially sharply in a vacuum furnace tube, and therefore the heat-transfer coefficient of the medium in the furnace tube is sharply reduced, which results in the decrease of the overall heat-transfer coefficient in the furnace. In order to achieve the same heat-transfer intensity, a temperature difference should be increased, that is to say, the temperatures of furnace box and furnace tube should be increased. This will lead to a too high topical temperature on the furnace tube wall and influence the use life of the furnace tube.
According to the results of the simulation and calculation, in the furnace tube of the radiation section of the vacuum heating furnace and in the oil transfer line, the flow rate of the vapor phase entrapped with large liquid droplets are very high, the mass transfer area between the vapor and liquid phases is relative small, therefore lighter fractions cannot be completely vaporized and enwrapped in the non-vaporized heavy oils, which results in that the real vaporization ratio of the feedstock coming into the vaporization section of the distillation column is lower than the equilibrium vaporization ratio calculated by the theory. A part of lighter components are present in the column bottom residual oil, and therefore the distillation yield will be decreased. Now, as to the domestic atmospheric and vacuum distillation plants, the design cut point for the vacuum residue is generally set at 540° C. In many vacuum residues, the fraction below 500° C. has a content of greater than 8 wt %; the fraction below 538° C. has a content of greater than 10 wt %, even as high as greater than 30 wt % for some vacuum residues. By example of the atmospheric and vacuum distillation unit of SINOPEC Hainan Refinery, the equilibrium vaporization ratio of the atmospheric residue at the temperature and pressure of the vaporization section of the vacuum distillation column is 59.0 wt %, however, the industrial distillation yield is only 51.9 wt %. This shows that there is a certain gap between the industrial distillation yield and the equilibrium vaporization ratio. Therefore, the vacuum distillation still does not reach the equilibrium vaporization ratio, and there remains a great room for improving the distillation yield.